MERITS AND DEMERITS OF TWO-STROKE ENGINES OVER FOUR-STROKE ENGINES

1 Merits

1. A two-stroke engine gives twice as many power strokes as a four-stroke cycle engine at the same engine speed; therefore, a two-stroke engine of the same size should develop twice the power of a four-stroke engine. In practice, the actual power developed by a two-stroke engine is about 1.7 to 1.8 times the power developed by a four-stroke engine of the same dimensions and speed. This happens because some of the power is used for compressing the charge in the crank case and the effective stroke is less than the actual stroke.
2. For the same power developed, the two-stroke engine is much lighter, less bulky, and occupies less floor area. Therefore, it is more suitable for uses in marine engines and transport purposes.
3. It provides mechanical simplicity as valves, rocker arms, push-rods, cam, and cam shafts are not required. The friction loss is also less, and therefore, it gives higher mechanical efficiency.
4. The two-stroke engines are much easier to start.
5. A crankcase compression and valve-less type two-stroke engine can run in either direction, which is useful in marine applications.
6. The initial cost of the engine is considerable less.
7. The weight/kW ratio is considerably less.

2 Demerits

1. The thermodynamic efficiency of an engine is only dependent on the compression ratio. The effective compression ratio for a two-stroke engine is less than that for four-stroke engine for the same stroke (actual) and clearance volume. Therefore, the thermodynamic efficiency of two-stroke cycle is always less than a four-stroke cycle engine.
2. The actual efficiency of a two-stroke cycle is less than a four-stroke cycle engine because greater overlapping of ports is necessary in a two-stroke engine for effective scavenging. A portion of fresh charge in the case of an SI engine always escapes unused through the exhaust ports; therefore, the specific fuel consumption is usually higher.
3. As the power-strokes per minute are twice the power stroke of four-stroke cycle engines, the capacity of the cooling system used must be higher. The cooling of the engine also presents difficulty as the quantity of heat removed per minute is large. Due to firing in each revolution, the piston is likely to get overheated and oil cooling of the piston is necessary.
4. The consumption of lubricating oil is sufficiently large because of high operating temperatures.
5. Sudden release of the gases makes the exhaust noisier.
6. A two-stroke petrol engine with crankcase compression 50–60% of the swept volume is filled with fresh charge, whereas a four-stroke petrol engine contains 80–95%. This happens because the space occupied by the rotating parts in the crankcase prevents a full charge being sucked in.
7. The scavenging is not complete, particularly in high speed engines, as very short time is available for exhaust; hence, the fresh charge is highly polluted. This can be reduced using an opposed piston two-stroke diesel engine which provides unidirectional scavenging.
8. The turning moment of a two-stroke engine is more non-uniform as against with four-stroke engine, so it requires heavier flywheel and strong foundation.